1. Field of the Invention
The present invention relates to a fluidized bed heat generating system and a method of operation. In particular, the present invention is concerned with maintaining a selected particle size range for recirculating solid fine particulate material which is entrained in gaseous fluid flowing through a combustion reactor vessel which has a fluidized dense bed section at a lower level therein containing a permanent bed of large coarse solid particulate material. Combustion takes place in and above the dense bed and recirculating fine solid particulate materials move through the system absorbing and giving up heat generated in the combustion process.
2. Description of the Prior Art
The present invention is an improvement over fluidized bed systems such as those disclosed in U.S. Pat. Nos. 4,084,545 and 4,154,581 wherein a dense fluidized bed containing relatively coarse solid bed particles is maintained within a combustion vessel wherein entrained fluidized relatively find solid bed particles are recirculated to absorb heat generated in a combustion process taking place within the vessel adjacent the vicinity of the dense fluidized bed section. It has been found that prior art systems of the type disclosed in the aforementioned U.S. patents have had problems because of accumulation, degradation, attrition and agglomeration of the solid particulate materials used in the system.
Accumulation of the fine solid particles sometimes occurs in the dense bed section at times of operation at less than full or design primary airflow and when operating at reduced loads, particles in a size range of approximately 500 microns to 1000 microns have been difficult to entrain and move out of the fluidized dense bed section for recirculation.
Degradation of the dense bed material sometimes occurs as a result of thermal shock upon addition of make up to said dense bed. The results are similar to attrition but occur quickly, forming middling sizes which are ineffective as a grinding media for fuel and ineffective as time delay material for the fine entrained material. These middling sizes may accumulate in the dense bed at low fluidizing velocities or may leave the dense bed at high fluidizing velocities. Degraded or attrited material leaving the top of the dense bed act as a cloud adjacent the region of secondary air admission.
Attrition of the relatively coarse solid particulate materials in the dense bed section occurs and causes particles to be formed in a size range of approximately 1000 microns to 6000 microns and these particles tend to accumulate in the dense bed section. Such particles are too small in size and surface areas to efficiently operate in the dense bed environment.
Agglomeration of and growth of the smaller or fine solid particles in the dense bed section also occurs and this tends to defluidize the dense bed section with a marked increase in the pressure drop across the dense bed.
Another problem with present fluidized bed systems is the relatively poor performance at less than design loads caused because of an increased pressure drop in the dense bed section when the primary airflow is not modulated or reduced to accommodate reduced loads. As a result, part load performance is degraded because of high stack losses and high NOx production, both of which are caused by high excess air at low heat loads.
Because of attrition problems, the dense bed section produces middling size particles which are not large enough in surface area for size reduction of the solid fuel and which cause too high a pressure drop in the dense bed section, thereby resulting in increased power consumption, even at normal design loads.